1. Field of the Invention
The present invention relates to methods for controlling centrifuges to improve solid-liquid separations, and particularly to achieve desired or optimal solids concentration in the concentrate discharge.
2. Description of the Prior Art
In certain industrial processes, intermittently discharged disk-stack centrifuges are used to carry out solid-liquid separations. A single stream, the feed, consisting of a mixture of an insoluble, sedimentable solid phase and a liquid phase, is continuously delivered to the centrifuge. The sedimentable solid phase is separated from the non-sedimentable liquid phase on the disks within the spinning bowl, and is flung to the periphery of the inside of the bowl, where it accumulates. At discrete intervals, the bowl is opened momentarily and the accumulated solids are discharged from the bowl.
The discharged material is referred to as the concentrate, because it is enriched in solid phase relative to the feed. Another exit stream, the centrate, is deprived of the solid phase relative to the feed. The product of interest may be in the liquid phase, such as in the clarification of alcoholic beverages or industrial fermentations in which the product is secreted into the medium. Alternatively, the product may be in the solid, sedimentable phase, as when a highly-expressed recombinant protein forms inclusion bodies intracellularly.
The operational parameters of an intermittently discharged disk-stack centrifuge are feed flow rate, shot interval, and shot duration. The term "shot" is a colloquial expression for the discharge event, stemming from the noise made on discharge which is similar to the firing of a weapon. Feed flow rate is the rate at which the feed is conveyed into the machine. The shot interval is the length of time between discharges of the concentrate. The shot duration is the length of time the bowl is opened during a discharge. The feed flow rate determines the residence time of solid particles in a centrifuge and the rate at which solids and liquid enter the machine. The shot interval and shot duration together determine the rate of removal of solids, and to some extent influence the concentration of solids in the discharged material.
The degree to which the interdependent process objectives of yield, purification and concentration of solids in the discharge are achieved is a measure of the performance of the intermittently discharged disk-stack centrifuge. Yield is a measure of the amount of product leaving the centrifuge in the stream which will be processed further compared to the amount of product entering the centrifuge. Purity is the ratio of the concentration of the product in a stream to the concentration of something else in the stream, such as nonvolatile solids or protein. Purification is the ratio of the purity of the product in the stream which will be processed further, compared to the purity of the product in the feed stream. Purification is a measure of the degree of separation achieved in the centrifuge between product and contaminants.
The concentration of solids in the discharge is the performance variable most directly controllable via the operating parameters of the centrifuge. The process objectives of yield (for a liquid phase product) and purification (for a solid phase product) are not directly controllable, but depend somewhat on the concentration of solids in the discharged stream. For a liquid-phase product, the greater the proportion of the liquid that is discharged in the concentrate stream, the lower is the yield of product in the centrate stream. In order to maximize the yield, no liquid would be allowed to be discharged in the concentrate stream. However, if no liquid is included in the discharge, the resulting concentrate is a very thick paste that cannot be discharged. Thus, in order for the solids phase to be conveyed out of the machine at discharge, some volume of liquid must be discharged with the solids. Optimized performance with respect to yield for a liquid-phase product requires, then, that the amount of liquid discharged with the solids be just sufficient to prevent the discharged slurry from forming a paste that cannot be conveyed out of the machine. That is, the discharge should be just thin enough to be discharged efficiently.
To optimize the purification of a solid-phase product, the amount of liquid in the concentrate stream should be no more than is necessary to allow efficient discharge of the concentrate. This is because soluble material in the liquid will reduce purity. Achieving an optimal concentration of solids in the concentrate is essential for optimized centrifuge operation. Additionally, it is desirable to achieve a particular target solids concentration in the concentrate when the next processing step has a low tolerance for liquid, is limited in its volume handling capacity, or benefits from low solids concentration variability. In summary, good control of the solids (or liquid) content of the discharged stream is essential when the objectives are to optimize yield for a liquid-phase product, to optimize purification for a solid phase product, or to produce a discharge having a high solids concentration or a solids concentration in a narrow range.
In the past, the operating parameters to control the amount of liquid in the discharged concentrate were chosen empirically at the beginning of processing, and then altered during the separation in response to in-process measurements. The performance of the centrifuge during operation was monitored by periodically removing a sample of the concentrate stream and measuring the volumetric percentage of solids using a small centrifuge (a Gyro tester). If the volumetric percentage of solids in the discharge deviated from a target value, then the centrifuge operating parameters were varied by trial-and-error until the measurement came within a prescribed range of the target.
Sometimes this approach succeeded and the solids content of the concentrate met process expectations. Frequently, however, the process objective for solids content was not achieved, resulting in high variability in solids content and consistently suboptimal purification. The most important causes of this high variability were the imprecision in the in-process measurement of volumetric percent solids, and the lack of a model by which process objectives (solids content, yield, purification), the operational parameters of the centrifuge (feed flow rate, shot interval, and shot duration), in-process measurements, and characteristics of the feed stream could be related.
The present invention provides a method for controlling the centrifuge that greatly increases the likelihood of producing a concentrate discharge having a desired or optimal solids concentration. This method is based on mathematically sound mass balances and a model to relate process objectives, the operational parameters of the centrifuge, in-process measurements, and the characteristics of the feed stream.